1. Field of the Invention
The present invention relates to air purification systems for manufacturing cleanrooms, and in particular to an air purification system which utilizes acidic mist and alkaline mist.
2. Description of the Related Art
Generally, integrated circuit manufacturing involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placement of a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal, and other circuit-contamination sources.
In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers is getting more and more important as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer compared to larger devices in the past. Moreover, the presence of particles in the integrated circuit affects the functional integrity of the devices in the finished electronic product. To achieve an ultraclean wafer surface, particles must be removed from the environment, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors.
Because minimization of particles on wafers throughout the IC manufacturing process is critical, the environment within which the IC manufacturing process is carried out must be subjected to stringent controls of airborne particles which would otherwise enter the manufacturing environment from the sources outside the IC manufacturing facilities. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm. Accordingly, modern semiconductor manufacturing is carried out in a complex facility known as a cleanroom. The cleanroom is isolated from the outside environment and subjected to a stringent control of contaminants including airborne particles, metals, organic molecules and electrostatic discharges (ESDs), as well as environmental parameters such as temperature, relative humidity, oxygen and vibration. Along with a sophisticated system of filters and equipment, a comprehensive and strictly-enforced set of procedures and practices are imposed on facility personnel in order to maintain a delicate balance of these clean air requirements and parameters for optimal IC fabrication.
A modern cleanroom used in the fabrication of integrated circuits includes one large fabrication room having a service access corridor that extends around the perimeter of the cleanroom and a main manufacturing access corridor that extends across the center of the cleanroom. Production bays, which accommodate the semiconductor fabrication tools, are located on respective sides of the main manufacturing access corridor. The outside air enters the cleanroom through an air purification system, which is located above the ceiling of the cleanroom and includes particulate filters, typically HEPA (high-efficiency particulate air) filters. Through openings in the ceiling, the air is drawn downwardly in a continuous laminar flow path from the air purification system, through the cleanroom and openings in the floor into an air recirculation system. The air recirculation system may turn the air over every six seconds in order to achieve ultraclean conditions during disturbances such as changes in personnel shifts. An exhaust system removes heat and chemicals generated during the fabrication processes.
U.S. Pat. No. 6,059,866 teaches an air scrubber that eliminates dust or harmful gases in air, reduces the supply of pure water used as clean water, and performs humidification with high saturation efficiency. Moreover, U.S. Pat. No. 6,387,165 teaches an airborne molecular contaminant removal apparatus applied to remove airborne chemical contaminants such as particulate contaminants or airborne molecular contaminants from the outside air.
FIG. 1 is an example of a conventional air purification system 10 used to purify outside air 38 drawn through the system 10 and into a semiconductor fabrication facility cleanroom (not shown). The air purification system 10 includes an elongated housing 12 having an intake end 14. A blower 18 pumps the outside air 38 into the intake end 14 and then initially through a pre-filter 16 in the housing 12, where pre-filter 16 removes particles from the air 38 if the particles are larger than a selected size. The air then flows through an upstream cooling coil 20, which cools the air to a temperature at or below the dew point in such a manner that moisture in the air coalesces into water droplets 44, as shown in FIG. 2. Multiple nozzle conduits 22, each equipped with multiple spray nozzles 24, are provided in the housing 12. Each nozzle 24 generates a water spray 25 that contains a fine mist of additional water droplets 44. The water droplets 44 are bound with the airborne particles 42 and carry the particles 42 to a high density eliminator 26, made of high-density paper or non-woven cloth. A downstream cooling coil 28 is provided on the opposite side of the high density eliminator 26. As shown in FIG. 2, many of the particles 42, bound to the water droplets, accumulate on the high density eliminator 26. Most of the droplet-bound particles 42 are eventually pulled down by gravity and are collected in collection pan 46 at the bottom of the housing 12. The air, from which most of the airborne particles 42 have been removed, next flows through a low density eliminator 30, made of a low-density fabric curtain material, and then, through a heating coil 32 which heats the air to room temperature. Before exiting the outlet end 48 of the housing 12, the air passes through a chemical filter 34, which removes chemical residues from the air, and through a HEPA filter 36. The HEPA filter 36 is a high-efficiency filter which removes about 99.98% of the airborne particles from the air flowing therethrough. Finally, the purified air 40 emerges from the outlet end 48 of the housing 12 and enters the cleanroom (not shown) of the facility through a suitable air distribution system (not shown).
In FIG. 1, the conventional air purification system 10 uses a large quantity of water to catch and remove the AMCs (Airborne Molecular Contaminants from the outside air, the pH or conductivity value of water used in the conventional air purification system 10 must be precisely controlled at specify value, such as 6.2 or 10 us/cm When the conductivity or pH value of spray water changes, the molecule removing efficiency drops. Thus, the conventional air purification system hard to get better removal efficiency for AMC removal. So we need an additional chemical filter 34 to remove chemical residue from the air. Accordingly, a new and improved air purification system is needed to achieve a better acidic or alkaline molecule removing efficiency without any precise pH value control.